1. Field of the Invention
The present invention relates to a hydraulically lockable gas spring for continuous height adjustment of objects, particularly of drawing-tables.
2. The Prior Art
It is known in the prior art to provide hydraulically lockable devices in the support columns or legs of drawing tables or other objects which carry weight, e.g., chairs. Such prior art devices typically include a liquid-filled cylinder, a piston rod sealingly guided for movement inward and outward of said cylinder and being connected to a piston within the cylinder. The piston rod is connected to the object to be supported, and the cavity of the cylinder is connected to a second liquid chamber, which is subject to the pressure of a pressurized gas contained in a gas chamber, via an externally controlled locking member.
A gas spring of the above type including hydraulic locking means has been known from German Offenlegungsschrift No. 1,554,251. The hydraulic locking is effected by the bottom valve which interconnects the cylinder cavity filled with liquid and the second chamber filled with liquid. A chamber filled with pressurized gas and provided adjacent the second liquid chamber exerts a force on the piston rod when the bottom valve is opened which substantially corresponds to the weight which is exerted on the gas spring by the object to be vertically adjusted. It is a disadvantage of this construction that an increase in weight of the member to be vertically adjusted--for example, by heavy articles deposited on a drawing-table--causes the member which is vertically adjusted suddenly to slide down. In particular, that is the case with vertically adjustable drawing-tables whose weight is increased by additional appliances, so that the user can be exposed to danger.
In connection with drawing-tables which are in the form of a one-column frame, it has been known from German Auslegeschrift No. 2,513,128 to prevent the danger to the user by providing additional mechanical brakes to prevent a release of the locking means in case of greatly increased weight of the drawing-table plate, and also to lock the gas spring mechanically in the case of low gas pressure in the spring. Such a combination of a gas spring, a mechanical brake, and a safety means preventing drop of the plate is very complex in manufacture and moreover requires a precise adjustment in order to adapt the moment of response of the drop brake to the respective weight of the drawing-table and to compensate for production tolerances.